But as for your question, Andy, sorry, I do not even know if my *limited* testpics collection contains any. I don't understand why you bother with thumbnails anyway, today's processing power
is sufficient to simply read the whole file and present your user with a preview from that.

You are aware that people use TIFF for things like 3600dpi film scans, right?

If the choices are a "space-wasting" (several to a couple tens of kilobytes!) thumbnail image and making the user wait more than a few seconds to see the data in the file they're working with,
the choice is pretty clear from an interface perspective.